#include <stdio.h>

int main()
{
    int score;
	scanf("%d",&score);
	score/=10;
	
	switch(score){
		case 10:
		case 9:
		   printf("A\n");
		   break; 
		case 8:
		   printf("B\n");
		   break;
		case 7:
		   printf("C\n");
		   break;
		case 6:
		   printf("D\n");
		   break;
		default:
		   printf("F\n");
		   break;
    }
		return 0;
}
